Integrated circuits are made by photolithographic processes, which use photomasks or reticles and an associated light source to project a circuit image onto a wafer. The presence of defects on the surfaces of the photomasks, reticles, or wafers are highly undesirable and adversely affect the operation of the resulting circuits. The defects can be due to, but not limited to, a portion of the pattern being absent from an area where it is intended to be present, a portion of the pattern being present in an area where it is not intended to be, chemical deposition or residues from the photomask manufacturing processes which cause an unintended localized modification of the light transmission property of the photomask, artifacts in the photomask substrate such as pits, scratches, and striations, and localized light transmission errors in the substrate or pattern layer.
Defects may also be caused by particulate contaminates such as dust, resist flakes, skin flakes, and erosion of the photolithographic pattern. Conventionally, masks have been protected from such contaminants via a pellicle that is interposed between the light source and the wafer. In most cases, the pellicle is disposed in relatively close proximity to the surface of the mask between the source of light and the mask. Unfortunately, however, the optical components used to distribute the light from the light source are left unprotected.
Although certain types of defects may be prevented, there are many other types of defects that still tend to occur (as described above). These defects have to be found and repaired prior to use in order to maintain a high yield. Methods and apparatus for detecting defects have been around for some time. For example, inspection systems and methods utilizing light have been introduced and employed to various degrees to inspect the surface of substrates such as photomasks, reticles and wafers. These inspection systems and methods generally include a light source for emitting a light beam, optics for focussing the light beam on the surface of the substrate, a stage for providing translational travel, collection optics for collecting either transmitted and/or reflected light, detectors for detecting either the transmitted and/or reflected light, and a means for constructing a virtual image of the substrate being inspected.
Although such inspection systems work well, there are continuing efforts to improve their designs, as for example, to protect the optical surfaces of the optical components used therein. As should be appreciated, particle contamination that adheres to the optical surfaces may adversely effect the optical quality of the inspection system thereby producing undesirable and/or unpredictable results.